Telecommunication connections are subject to various interferences. There are several reasons for interference. In telecommunication connections, the transmission path used for transmitting signals is known to cause interference to telecommunication. This occurs regardless of the physical form of the transmission path; whether the transmission path is, for instance, a radio link, an optical fibre or a copper cable. Particularly in radio telecommunication there are frequently situations where the quality of the transmission path varies from one connection to another and also during a connection.
Radio path fading is a typical phenomenon that causes changes in a transmission channel. Other simultaneous connections may also cause interferences and they can vary as a function of time and place.
To reduce effects of various interferences, several solutions have been evolved. This is the case in radio communication in particular. Various telecommunication methods have been evolved, aiming at achieving as high spectral efficiency as possible and still minimising the effect of interference. A wireless telecommunication method which has been very much studied lately is a wide-band spread-spectrum multiple access method CDMA (Code Division Multiple Access).
Compared to multiple access methods that were generally used earlier, the CDMA has a plurality of advantages. In a CDMA method, a narrow-band data signal of the user is multiplied by a spreading code having a considerably wider band to a relatively wide band. Bandwidths used in known experimental systems include 1.25 MHz, 10 MHz and 25 MHz. In the multiplying process, the data signal spreads to the entire band used. All users transmit simultaneously by using the same frequency band. Each connection between a base station and a mobile station uses its own spreading code, and the signals of the users can be distinguished from each other in the receivers on the basis of the spreading code of each user. The aim is to select the spreading codes so that they are mutually orthogonal, i.e. they do not correlate with each other. The above described CDMA method is called a direct-sequence method, DS-CDMA (Direct Sequence CDMA). There are other CDMA methods as well, such as a frequency hopping method FH-CDMA (Frequency Hopping CDMA), in which the frequency used is varied quickly according to the used spreading code. A combination of these methods is also possible.
A problem of wide-band data transmission is narrow-band interference, which is typically caused by narrow-band signal sources external to the system, using the same or overlaying frequency band with the system. Typical of these interference signals is that their properties and structure often differ from the signals of the system considerably. They often use a lower data transmission rate. Also, it is often so that the properties of interfering signals are not known in advance.
The CDMA can, to some extent, inherently compensate the effect of narrow-band interference on wide-band data transmission, but if the interfering signal is much stronger than the wide-band signal, it may cause considerable interference.
To suppress narrow-band interference from a wide-band signal, various methods have been evolved. The publication S. Sandberg, “Adapted Demodulation for Spread Spectrum Receivers which Employ Transform-Domain Interference Rejection”, IEEE Trans. On Communications, Vol. 43, pp 2502–2510, September 1995, discloses a method in which the problem is approached in the frequency domain, and the interference is suppressed by removing the frequencies where the interference is assumed to be. In the presented solution, however, the location of the interference in the frequency band and the bandwidth of the interference are assumed to be known. The interference is also assumed to be stationary.
The publication M. Lops, A. Tulino, “Automatic Suppression of Narrowband Interference in Direct Sequency Spread-Spectrum Systems”, IEEE Trans. On Communications, Vol. 47, pp. 1133–1136, August 1999, discloses a method in which non-stationary interference, the properties of which are not known, can be suppressed. However, the achieved results are not qualitatively satisfactory.